How to Use the Command 'nix-shell' (with Examples)

The nix-shell command is a powerful tool from the Nix package manager that allows users to spawn an interactive shell environment based on a Nix expression. It is particularly useful for development environments or any scenario where you need a specified set of dependencies temporarily available without affecting the primary system configuration. The flexibility of nix-shell lies in its ability to manage dependencies and environments independently, ensuring reproducibility and consistency.

Use Case 1: Start with Nix Expression in shell.nix or default.nix in the Current Directory

Code:

nix-shell

Motivation:
When working on a project, you often need a specific environment that includes various tools, libraries, and configurations. By specifying these in a shell.nix or default.nix file in your project directory, you can easily recreate that environment on any machine simply by running nix-shell. This ensures that all team members or build environments have exactly the same setup.

Explanation:

• nix-shell: This runs the command using the Nix expression found in the current directory’s shell.nix or default.nix. It reads the dependencies and settings listed in the file and sets up the shell environment based on those specifications.

Example Output:

[nix-shell:~/project]$  

You are dropped into a shell with all the dependencies and environment variables as specified in your shell.nix or default.nix file.

Use Case 2: Run Shell Command in Non-Interactive Shell and Exit

Code:

nix-shell --run "command argument1 argument2 ..."

Motivation:
Sometimes you just need to run a command with a specific set of dependencies without entering an interactive shell. This is useful for running scripts or tools within a defined environment, especially in automation scripts or CI/CD pipelines where interactivity is not needed.

Explanation:

• --run: This option tells nix-shell to execute the command provided as a string and exit immediately after it’s done, without starting an interactive shell.
• command argument1 argument2 ...: Replace with your specific command and its arguments. The command will run in the environment set up by nix-shell.

Example Output:

Hello, Nix!

This would be the output if your command was echo "Hello, Nix!".

Use Case 3: Start with Expression in default.nix in the Current Directory

Code:

nix-shell default.nix

Motivation:
In some cases, you specifically want to ensure that the shell environment is created using a particular Nix expression file, ignoring any shell.nix that might exist. This is particularly useful in monorepo setups where different parts of the repository might require different environments.

Explanation:

• default.nix: The command directly specifies default.nix as the Nix expression file to be used for setting up the environment, overriding any implicit searching or defaults.

Example Output:

[nix-shell:~/project]$

You’re now in an environment specified by the default.nix file.

Use Case 4: Start with Packages Loaded from Nixpkgs

Code:

nix-shell --packages package1 package2 ...

Motivation:
Sometimes all you need is to quickly have access to some packages that are available in the Nixpkgs repository without setting up a full shell.nix or default.nix file. This is perfect for ad-hoc development or testing tasks where you might need access to tools like git, python, etc.

Explanation:

• --packages: This option allows you to specify which packages to install from the Nixpkgs repository for the shell session.
• package1 package2 ...: Replace with the specific package names you’d like to have available.

Example Output:

[nix-shell:~/project]$ git --version
git version 2.30.0

This output indicates that the Git package is available within your Nix shell environment.

Use Case 5: Start with Packages Loaded from a Specific Nixpkgs Revision

Code:

nix-shell --packages package1 package2 ... -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/nixpkgs_revision.tar.gz

Motivation:
Different projects might need different versions of the same package due to varying requirements or compatibility issues. By specifying a Nixpkgs revision, you can ensure that your development environment uses the exact versions of packages that your project depends on, maintaining consistency.

Explanation:

• --packages: Specifies the desired packages to load.
• -I nixpkgs=...: Points to a specific revision of the Nixpkgs repository, allowing you to load packages exactly as they were at the time of that revision.
• nixpkgs_revision: Placeholder for a specific commit hash or tag to use from the Nixpkgs repository.

Example Output:

[nix-shell:~/project]$ python --version
Python 3.9.0

The exact version reflects what’s available at the specified Nixpkgs revision.

Use Case 6: Evaluate Rest of File in Specific Interpreter for Use in #!-scripts

Code:

nix-shell -i interpreter --packages package1 package2 ...

Motivation:
Creating #!-scripts that work in any environment by specifying their dependencies through Nix is extremely powerful. This use case is prevalent for scripts that need a specific version of an interpreter or libraries to ensure that they work correctly regardless of the user’s underlying system configuration.

Explanation:

• -i interpreter: This flag tells nix-shell to use the specified interpreter, like bash, python, or another language interpreter, for running the remainder of the script.
• --packages: Specifies any dependencies required by the script, ensuring they are available when the script is executed.
• package1 package2 ...: The list of all packages needed for the script to run.

Example Output:

Script executed successfully.

After executing a script with the environment set up, you observe the intended outputs or results.

Conclusion:

The versatility of nix-shell shines in various scenarios, ranging from setting up complex development environments to running simple scripts with specific dependencies. By understanding these use cases, users can leverage nix-shell to improve reproducibility, consistency, and reliability in their workflows. Whether working individually or in a large team, these examples highlight how nix-shell can be tailored to meet diverse development needs.